1. Field of the Invention
The present invention relates to a light modulation panel and an imaging lens. Particularly, the present invention relates to a light modulation panel for making an imaging lens composed of an image-forming optical system function as a depth-of-field-extended optical system, and to the imaging lens in which the light modulation panel is mounted.
2. Description of the Related Art
Conventionally, techniques for correcting a blur in an original image obtained by imaging an optical image to which a blur has been through a depth-of-field-extended optical system are known. The blur is corrected by performing image restoration processing on the blurred original image.
In the image restoration processing, a restoration filter the properties of which are opposite to the blur characteristics of the depth-of-field-extended optical system is applied to the original image to increase the contrast of the original image. Accordingly, an image that would be obtainable by using an imaging lens having a deep depth of field is generated.
When the technique for extending the depth of field is applied, for example, to a monitor or surveillance camera for monitoring the inside of a store or the like, it is possible to monitor a wide range of photography from a short distance to a long distance.
Further, a technique for making an imaging lens composed of an image-forming optical system function as a depth-of-field-extended optical system by inserting a light modulation panel into an optical path of the imaging lens is known. The image-forming optical system is an optical system that forms an optical image representing a subject in such a manner that various aberrations are minimized.
Further, an imaging lens that is composed of an image-forming optical system, and that functions as a depth-of-field-extended optical system when a light modulation panel is inserted into an optical path of the imaging lens, but functions as an image-forming optical system when the light modulation panel is removed therefrom is known (please refer to U.S. Patent Application Publication No. 2010110233). In other words, the imaging lens composed of the image-forming optical system is convertible to the depth-of-field-extended optical system, and vice versa.
When the imaging lens convertible to the image-forming optical system and to the depth-of-field-extended optical system, as described above, is applied to a monitor camera, it is possible to switch operations of the monitor camera based on photography conditions. Specifically, operations are switchable between the case of imaging a subject located at a specific photography distance at extremely high resolution and the case of monitoring a wide range of photography from a short distance to a long distance including the subject at an acceptable level of resolution.
However, when a light modulation panel is simply added to an imaging lens that functions as an image-forming optical system, the imaging lens does not always function as a depth-of-field-extended optical system. In some cases, a desirable image restoration effect is not achievable even if image restoration processing is performed on an original image imaged through the imaging lens into which the light modulation panel has been inserted.
For example, when a light modulation panel is inserted to an optical path of an imaging lens composed of a fixed-focus image-forming optical system to make the imaging lens function as a depth-of-field-extended optical system, if the position of an image plane with respect to the imaging lens is not adjusted in an appropriate manner, it is not always possible to obtain an original image from which a desirable image restoration effect is achievable. For example, when image restoration processing is performed on an original image obtained by imaging a subject located at a short distance and a subject located at a long distance at the same time, an image having a desirable contrast may be restorable with respect to the blurred original image representing the subject located at the long distance but an image having a desirable contrast is not always restorable with respect to the blurred original image representing the subject located at the short distance.
Such a phenomenon occurs, because a position at which an optical image should be imaged when the optical image is formed when the imaging lens functions as the depth-of-field-extended optical system by insertion of the light modulation panel into an optical path of the imaging lens (in other words, the position of an image plane at which an original image from which a desirable image restoration effect is achievable can be obtained) differs from an image formation position of an optical image formed through only the imaging lens composed of the image-forming optical system.
Therefore, there is a demand for imaging an original image from which a desirable image restoration effect is achievable without adjusting an optical system when the optical system is converted from an image-forming optical system without insertion of a light modulation panel into an optical path of the imaging lens to a depth-of-field-extended optical system with insertion of the light modulation panel into the optical path of the imaging lens.
The aforementioned problem is not specific to an imaging lens used in a monitor camera but common to imaging lenses used in general cameras.
For example, with respect to general digital cameras, there is a demand for easily switching operations between the case of obtaining an image representing only a person or people at an extremely high resolution and the case of obtaining an original image from which an image having a deep depth of field, and which represents both a person or people and the background at an acceptable level of resolution, is restorable.